Gaseous fuels can be stored at cryogenic temperatures when employed as fuel for internal combustion engines. A gaseous fuel is defined as any fuel that is in a gas state at standard temperature and pressure which is defined herein as 1 atmosphere and between 20 and 25 degrees Celsius. The gaseous fuel is stored near its boiling point in a storage vessel. For example, for methane at a storage pressure of about 1 atmosphere it can be stored in liquefied form at a temperature of about −161 degrees Celsius. Natural gas is a mixture of gasses with methane typically comprising the largest fraction, storage temperature can vary, but is normally close to that of methane. From the storage vessel the liquefied gas is pumped in a liquid state towards and through a heat exchanger where the temperature of the gaseous fuel is increased. Depending upon gaseous fuel pressure at the inlet of the heat exchanger, the gaseous fuel is normally in either the liquid state or supercritical state as it enters the heat exchanger, and either the supercritical state or gas state as it leaves. It is also possible that the gaseous fuel can be in the two phase state when the temperature and pressure are at or near the critical point. There are advantages to storing the gaseous fuel in a liquefied state. The density increases when the gaseous fuel is in the liquid state compared to either the supercritical state or the gas state requiring a smaller volume to store an equivalent amount of fuel on an energy basis. Since liquids are relatively incompressible compared to gases, it is more efficient to pressurize a gaseous fuel when in the liquid state compared to the gas state. After vaporization in the heat exchanger a fuel injection system receives vaporized gaseous fuel and introduces it, either directly or indirectly, to one or more combustion chambers in the engine. As used herein, vaporizing refers to at least increasing the enthalpy (that is, temperature) of the gaseous fuel as it pass through the heat exchanger, and depending upon the pressure and the temperature of the gaseous fuel it can also refer to changing the state of the gaseous fuel to the gas state. While natural gas (LNG) is an exemplary gaseous fuel, which is employed in many high horse power (marine, mining, locomotive) and heavy duty engine applications, other gaseous fuels are equally applicable to the technique described herein.
A heat source is required in the heat exchanger to increase the temperature of the gaseous fuel above its boiling point. Engine coolant from the water jacket of the internal combustion engine can be employed as the heat source. The engine coolant is routed through a separate path in the heat exchanger such that waste heat from combustion is transferred to the liquefied gaseous fuel from the storage vessel causing it to vaporize. By employing waste heat from the combustion process efficiency is improved compared to employing energy derived from the engine output, for example such as electrical energy from a generator driven by the engine.
It is important to control the temperature of the gaseous fuel discharged from the heat exchanger for a number of reasons. First, the gaseous fuel discharged from the heat exchanger is normally required to be in a particular state, for example the supercritical state. Second, the temperature must be above a predetermined minimum value such that components downstream from the heat exchanger are protected from excessively cold temperatures that may cause component failure. When the temperature of gaseous fuel downstream of the heat exchanger drops below the predetermined minimum value, or if it is predicted to drop below the predetermined minimum value, then the pump transferring gaseous fuel from the storage vessel to the heat exchanger must be suspended (stopped). Delivery of gaseous fuel to the fuel injection system stops when the pump stops and available fuel injection pressure decreases below the requisite level as the engine continues to consume fuel. As available fuel injection pressure decreases the engine can be designed to continue operation with a derated power output and then eventually stop, or go to a back-up secondary fuel. This situation is not desirable.
It is possible for the temperature of gaseous fuel discharged from the heat exchanger to decrease below the predetermined minimum value when the engine coolant is too cold, or when the residence time of the gaseous fuel inside the heat exchanger is too short, or due to a combination of these two reasons. During normal engine operating conditions engine coolant temperature is maintained between a predetermined range. However, engine coolant temperature can deviate from this range for a variety of reasons. One such reason is cold start of the engine when engine coolant temperature is equivalent or near to ambient temperature, which is much lower than engine coolant temperature during normal engine operating conditions. Excessively cold ambient temperatures may also cause engine coolant temperature to drop below the predetermined temperature range, or at least worsen cold start performance.
The volume of gaseous fuel inside the heat exchanger can be less than the maximum displacement volume of the pump, although this is not a requirement. For example, in high pressure direct injection applications where diesel is employed as a pilot fuel, the volume of gaseous fuel inside the heat exchanger is normally less than the maximum displacement volume of the pump since the engine can be fuelled with diesel on start-up and as the engine coolant warms up the engine can switch over to the gaseous fuel. In low-pressure spark-ignited applications, where the gaseous fuel is port or manifold injected, the volume of gaseous fuel inside the heat exchanger is normally greater than the maximum displacement volume of the pump since the engine is fuelled with gaseous fuel immediately on start-up and more residence time is needed for the gaseous fuel inside the heat exchanger to increase the enthalpy of the gaseous fuel when the engine coolant temperature is below normal engine operating conditions. Under normal engine operating conditions the temperature differential between engine coolant and the liquefied gaseous fuel inside the heat exchanger is sufficient to completely vaporize the gaseous fuel discharged from the heat exchanger. However, when the engine coolant is too cold the residence time of the gaseous fuel inside the heat exchanger is insufficient to effect its vaporization. Depending upon engine operating conditions, there is a need to increase the residence time of the gaseous fuel inside the heat exchanger to increase its temperature above a predetermined value, whether the volume inside the heat exchanger is greater than or less than the maximum displacement volume of the pump.
One technique to increase residence time of the gaseous fuel inside the heat exchanger is to decrease pump speed. However, there is a corresponding decrease in the flow rate of gaseous fuel when pump speed is decreased, which can cause fuel pressure downstream of the heat exchanger to drop or cause unwanted fuel pressure fluctuations. Normally, the engine is not running at full load and the pump does not need to be stroking continuously without suspension. It is possible under these conditions to decrease pump speed to increase residence time of the gaseous fuel in the heat exchanger. However, in systems where the pump is directly driven from the engine it is not possible to change pump speed apart from a change in engine speed.
Canadian Patent No. 2,809,495 co-owned by the Applicant and issued Jun. 3, 2014 to Batenburg et al., hereinafter Batenburg, discloses a technique of controlling the temperature of a fluid discharged from a heat exchanger. A cryogenic pump is operated in a plurality of modes. In a first mode, the cryogenic pump discharges a first amount of fluid from the heat exchanger in every pump cycle. The temperature that correlates to the fluid downstream from the heat exchanger is monitored, and when the temperature drops below a predetermined level the cryogenic pump is operated in a second mode where a second amount of fluid is discharged from the cryogenic pump for every pump cycle, where the second amount is less than the first amount. The residence time of the cryogenic fluid in the heat exchanger increases in the second mode, by pumping a smaller amount of fluid in every pump cycle, such that more heat is transferred to the fluid thereby increasing the discharge temperature. For each pump cycle, cryogenic liquid is drawn into a pumping chamber through an inlet check valve as a piston retracts during a suction stroke, and is then pumped through an outlet check valve by the piston as it extends during a compression stroke. In the second mode, no matter the quantity of cryogenic fluid that is discharged from the pump and vaporizer, the piston in the pump completes one suction stroke and one compression stroke, and the inlet and outlet check valves are open and closed respectively during the suction stroke and closed and open respectively during the compression stroke. As the inlet check valve opens cryogenic liquid fills the volume between the piston and cylinder head of the pump when the pump begins the intake stroke, and there is a corresponding pressure decrease in the cryogenic liquid in the pump. As the cryogenic liquid comes into contact with the interior walls of the pump and the piston it absorbs heat. Due to the pressure decrease and heat absorption, a portion of the cryogenic flashes each time the inlet check valve is opened, which reduces the volumetric efficiency of the pump. It is desirable to minimize the number of pumping cycles required to pump a predetermined mass of LNG to reduce the amount of LNG that flashes within the pump to increase the volumetric efficiency of the cryogenic pump.
There is a need for an improved technique of operating a cryogenic pump when controlling the discharge temperature of a vaporizer. The present method and apparatus provide a technique for improving operation of an internal combustion engine fuelled with a liquefied gaseous fuel.